Due both to demographic change and to developments in medical science; the number of surgical procedures involving prosthesis implantation is rising rapidly. The more obvious examples of prosthetic devices are hip or knee replacements and false teeth. Other less well-known examples are stems, heart valves, bone screws and plates and spinal fixators.
Prosthesis must be tolerated by the patient and not altered in time. Materials that may be suitable for each type of prosthesis are subjected to precise specifications. Indeed, if the prosthesis is a dental implant or a hip replacement the specifications will be very different. The most important requirements are mechanical properties similar to those of bone to allow the transfer constraints between bone and prosthesis, chemical resistance to corrosion, chemical inertia in relation to the environment and biocompatibility. These properties must be controlled to maintain the integrity of used materials. The human body is an aggressive and corrosive environment mainly because of concentrations of chloride ions (113 mEq/l in blood plasma and 117 mEq/l in the interstitial fluid, which is sufficient to corrode metallic materials) and dissolved oxygen. For dental implants, conditions are even tougher since the saliva contains more sulfur products that make it still more corrosive. The term “biocompatibility” is defined by the Dorland's Medical Dictionary as the quality of not having toxic or injurious effects on biological systems. This encompasses both the material and host responses to an implant. The host response to an implant can be highly complex and is often linked to the material response. It is also dependent on the anatomical position of the implant. For a material to be biocompatible, it should not elicit any adverse host reactions to its presence. Inflammation and encapsulation phenomena may occur when the prosthesis suffer from low biocompatibility.
Typically, prosthetic devices are made of inorganic (metal, alloys, ceramic and glass) and/or polymeric materials.
It is a fact that most pure metals and alloys are chemically unstable in many everyday environments due to their tendency to corrode. In the complex environment of the human body, metals and alloys are subject to electrochemical corrosion mechanisms, with bodily fluids acting as an electrolyte. While alloys such as stainless steel may appear to be highly stable and are widely used for kitchenware, eating utensils and jewelry, there are many situations that can cause severe corrosion of this material, and it is not the best choice for use in prosthetic devices.
Compared to inorganic materials, polymeric materials have certain advantages: they are lightweight, corrosion resistant, they can be directly shaped by molding and offer design freedoms. Over the past 4 years, the price of steel and non-ferrous metals grew faster than polymers and they require also less energy to be implemented. Among various existing polymers, only some of them have been used in the prosthesis industry so far, mainly because of their biocompatibility. Examples of such polymers are polymethyl methacrylate, polystyrene, poly(ether ether ketone) . . . .
Polyarylenes exhibit some outstanding performance properties, including exceptionally high strength, stiffness, hardness, scratch resistance, dimensional stability, great friction and wear properties, high solvent resistance and exceptional low temperature performance. These properties make them excellent candidates for end-use applications such as: mechanical components, aircraft interiors, coatings . . . .
In general, the so far known polyarylenes, while offering an exceptionally high level of strength and stiffness (which often even exceeds the needs of the applications wherein they are used) suffer irremediably from limitations in toughness-related properties:                they have limited elongability;        they have limited impact resistance (as typically characterized by standard notched and unnotched Izod tests).        
They also have also limitations in melt processability due to their high viscosities, and tend to be anisotropic when melt fabricated under high shear such as during injection molding. Also, they have some limitations in chemical resistance.
All materials used in the prior art prosthetic devices and known to the Applicant still suffer from a limited impact resistance, which is a key property for such application where such materials are submitted to various and harsh conditions.
There remains thus a strong need for a prosthetic device presenting a superior balance of properties, including part or preferably all of the following ones:                very high strength;        very high stiffness;        good elongation properties;        good melt processability (in particular, good injection moldability);        high chemical resistance;        good biocompatibility;        outstanding impact resistance, as possibly characterized by a standard no-notch IZOD test (ASTM D-4810).        
In fact, there is a specific need to improve the mechanical properties of the existing prosthetic devices and in particular their impact resistance while at least maintaining their biocompatibility.
The Applicant has found that a first specific type of polyarylene, taken alone or in combination with a second specific type of polyarylene offer surprisingly and in particular a good biocompatibility and impact resistance. This outstanding property may be useful in certain demanding applications, such as articles used as prosthetic devices.
It has been surprisingly found that the materials comprising:    (i) at least one polyarylene (P2) of another specific type, characterized by a “high” amount of kink-forming arylene units (up to 100%) such as Primospire® PR-250 polyphenylene, and optionally;    (ii) at least one polyarylene (P1) of a first specific type, characterized by a “low” amount (down to 0%) of kink-forming arylene units such as Primospire® PR-120 polyphenylene (formerly Parmax® 1200); are characterized by very good impact resistance properties. The intended meaning of the terms “low” and “high” will become clear in the light of what follows.
These outstanding properties have already been described in EP2007/052095, the content of which is incorporated by reference, which illustrates some of the surprising behaviors related to these specific materials.
The present invention is thus related to a prosthetic device comprising at least one part consisting of a material comprising at least one polyarylene (P2), of which the efficient arylene recurring units (R2) are a mix (M2) consisting of:                less than 70 mole %, down to 0 mole %, based on the total number of moles of efficient arylene recurring units (R2), of rigid rod-forming arylene units (Ra), said rigid rod-forming arylene units (Ra) being optionally substituted by at least one monovalent substituting groupwith        more than 30 mole %, up to 100 mole %, based on the total number of moles of efficient arylene recurring units (R2), of kink-forming arylene units (Rb), said kink-forming arylene units being optionally substituted by at least one monovalent substituting group.        
The present invention is also related to a prosthetic device comprising at least one part consisting of a material comprising:
at least one polyarylene (P2), of which the efficient arylene recurring units (R2) are a mix (M2) consisting of:
                less than 70 mole %, down to 0 mole %, based on the total number of moles of efficient arylene recurring units (R2), of rigid rod-forming arylene units (Ra), said rigid rod-forming arylene units (Ra) being optionally substituted by at least one monovalent substituting groupwith        more than 30 mole %, up to 100 mole %, based on the total number of moles of efficient arylene recurring units (R2), of kink-forming arylene units (Rb), said kink-forming arylene units being optionally substituted by at least one monovalent substituting group.andat least one polyarylene (P1), of which the efficient arylene recurring units (R1) are a mix (M1) consisting of:        from 70 mole % to 100 mole %, based on the total number of moles of efficient arylene recurring units (R1), of rigid rod-forming arylene units (Ra), said rigid rod-forming arylene units (Ra) being optionally substituted by at least one monovalent substituting groupwith        from 0 to 30 mole %, based on the total number of moles of efficient arylene recurring units (R1), of kink-forming arylene units (Rb), said kink-forming arylene units being optionally substituted by at least one monovalent substituting group.        
The prosthetic devices of the present invention feature some unexpected advantages because of the materials of which they are made.